The present invention relates generally to apparatus for winding web material, such as a paper web, on a tubular core to obtain a web roll.
In particular, the invention relates to an improvement in web material winding apparatus which includes a first carrier roll forming a first nip with the web roll, a second carrier roll forming a second nip with the web roll, a weight roll acting on the web roll with a press force, and wherein at least one of the first and second carrier rolls is a driven roll for rotating the tubular core and the web roll wound thereon.
The greatest drawback in conventional web material winding apparatus of this type is that it is difficult to control the tension of the web material during the winding process. In particular, it is necessary to increase the tension of the web material at the beginning of the winding operation so that the inner portion of the web roll being formed will not be too soft to withstand the effects of the more tautly wound outer portion of the roll produced at a later stage of the operation. If the inner portion of the web roll is too soft, the forces exerted on it by the outer roll portion will cause so-called puckers to be formed in the wound web roll.
Arrangements are known in the art wherein the hardness of a web roll is controlled during the winding operation. In one such arrangement, the web roll is carried on two parallel carrier rolls, the load from the web roll being divided between the carrier rolls. To this end, carrier rolls having substantially the same diameters can be disposed at different horizontal levels or, alternatively, carrier rolls having different diameters can be used. It is also known in the art that a harder wound web roll is obtained when the winding is performed on a carrier roll having a smaller diameter than in the case where the winding is performed on the carrier roll with a larger diameter.
Regarding the state of the art, reference is made to Finnish Patent Application No. 820383, corresponding to U.S. Pat. No. 4,465,243, in which an arrangement for centerless winding of web material is disclosed. In that arrangement the web is subjected to a substantially uniform tension until the web roll being wound reaches a certain size. As the size of the web roll increases, the puckering o pinching described above becomes unavoidable.